Fluidic devices can have integrated fluid channels for directing and controlling the transport of fluids. Microfluidics, a miniaturized form of fluidics, has emerged as a new approach for improving the performance and functionality of such systems for chemical and biochemical synthesis, as well as chemical, biochemical, and medical analysis. Miniaturization and new effects in micro-scale promise completely new system solutions in these fields. Dimension reduction results in faster processes with reduced reagent and sample consumption rates. The small size scale also encourages parallel processing, in which more compounds can be produced and/or analyzed simultaneously. Massively parallel processing can speed DNA, RNA, protein, immunologic, and other tests to reduce time intervals for drug discovery and medical diagnosis. Currently, microfluidic based microanalysis systems for such applications typically have fluid channel dimensions on the order of tenths of millimeters to several millimeters, although future trends are to further reduce channel dimensions. Various microfluidic components have also been demonstrated on the same size scale, for example: micro-valves, micro-pumps, micro-flow sensors, micro-filters, micro-mixers, micro-reactors, micro-separators, and micro-dispensers, to name just a few. The book, FUNDAMENTALS AND APPLICATIONS OF MICROFLUIDICS by Nam-Trung Nguyen and Steven T. Werely, published by Artech House of Boston, U.S.A., in 2002 provides an overview of some microfluidic technologies and applications. FIGS. 1A and 1B illustrated exploded and assembled perspective views of a fluidic or microfluidic device composed of a body component 101 to which a cover component 103 is affixed. Body component 101 contains channels 102 and other fluidic or microfluidic components formed therein.
In fluidic systems, and microfluidic systems in particular, it is often desirable to have flowing liquid segments with sharply defined frontal and trailing boundaries along channels. Such sharply defined boundaries minimize spatial dispersion during fluid flow and allow for more precisely defined timing for synthesis and/or analysis operations in such systems. “Wicking” as a result of capillary action between a working fluid and containment walls of a fluid transport channel, and in particular at the edges where containment walls meet, can spread out both frontal and trailing boundaries of the working fluid. Wicking tends to be exacerbated when a fluid and a channel wall have a higher degree of affinity for one another, for example in the case of an aqueous solution and a hydrophilic surface.
Various prior art approaches have been used to implement wicking inhibitor structures or “traps” to reduce wicking in channels and/or other structures. U.S. Pat. No. 6,919,058, RETAINING MICROFLUIDIC MICROCAVITY AND OTHER MICROFLUIDIC STRUCTURES, issued to Per Andersson, et al. on Jul. 19, 2005 (hereinafter “Andersson”), describes a wicking trap as illustrated in top and side cross-sectional views in FIGS. 2A and 2B, respectively. A fluidic channel 201 is defined by side and bottom walls in a first microfluidic component 101. A top bottom wall is defined by a second, top cover, component 103, that is bonded to the first component. Section 202 designates a region where the side walls have been surface treated so that they have less affinity for a fluid to be transported, thereby reducing capillary action and associated wicking. One difficulty of this approach is that such surface treatment requires an additional fabrication step for the fluidic device that adds cost, and is often difficult. FIGS. 3A and 3B respectively illustrate top and side cross-sectional views of another wicking trap embodiment described by Andersson. In this embodiment, channel wall recesses 301 are formed in the sidewalls of channel 201. These recesses tend to reduce wicking by interrupting capillary action at their corners. FIGS. 4A and 4B respectively illustrate top and side cross-sectional views of another wicking trap described by Andersson. In this embodiment, sidewall protuberances 401 extend into channel 201. These protuberances likewise can reduce wicking by interrupting capillary action.
U.S. Pat. No. 6,776,965, STRUCTURES FOR PRECISELY CONTROLLED TRANSPORT OF FLUID issued to Wyzgol et al. on Aug. 17, 2004 (hereinafter “Wyzgol”), presents the anti-wicking structure illustrated by FIGS. 5A and 5B in respective cross-sectional top and side views. In this embodiment, the bottom wall of the structure has a series of steps formed therein to reduce wicking by interrupting capillary action at their edges.
FIG. 6 shows a cross-sectional side view (normal to direction of fluid transport in a channel) that illustrates a feature common to all of these prior art embodiments. Where a channel surface of a sidewall 801 (or other structure formed by a sidewall) intersects with a channel surface of a top wall 802 formed by a cover as described above, an edge is formed with a ninety degree angle.
A requirement for capillary action at an edge of a channel formed, in part, by the intersection of two surfaces is known as the “Concus-Finn Condition” (R. Finn, Equilibrium Capillary Surfaces, Springer-Verlag, New York, 1987; R. Finn, A note on the capillary problem, Acta Math., 132 (1974) 199-205), which can be stated that if the contact angle of a fluid on a surface, and a half-angle of a corner formed by the intersection of two such surfaces exceeds ninety degrees, then the fluid will not wick along the corner. FIGS. 7A and 7B illustrate contact angles ✓ for a fluid 601 in contact with a surface 602, for low affinity and high affinity fluid/surface combinations, respectively. Especially for the case of fluids comprising aqueous solutions in contact with hydrophilic surfaces (a high affinity combination) we would expect ✓ to be small, for example in the ninety degree to zero to ninety degree range. FIG. 8 illustrates fluid meniscus 702 at an edge formed by the intersection of two wall surfaces 701 and 702. The corner angle between the intersecting wall surfaces is  and the half angle is . The Concus-Finn Condition for no wicking in terms of these variable is: ✓+>90°.
All of the prior art embodiments discussed above have a corner angle of substantially 2=90° as discussed above in connection with FIG. 6. Therefore the Concus-Finn Condition for no wicking requires that the fluid/surface contact angle ✓ should be greater than 45°. This condition limits the extent to which a fluid and a fluidic surface can have an affinity for one another without wicking, thereby limiting the usefulness of the prior alt wicking structures discussed above.